Movement of an aircraft primary flight control surface such as an aileron, elevator or rudder, requires the application of a control moment or torque to the control surface both to deflect the control surface and to maintain it at the newly deflected position. In some aircraft, the control surface may be connected to a fixed airfoil such as a vertical or horizontal stabilizer, and may typically include smaller movable trim tabs. The trim tabs may be deflected to counteract the torque required to maintain the control surface at its new position thereby reducing the force required to be applied by the pilot or autopilot.
Some aircraft may have a movable horizontal stabilizer which has a much larger surface area than the corresponding elevator, and as a consequence generates much larger pitching moments when responding to pitch control commands. When the aircraft is in a flight condition requiring sustained elevator deflection to maintain that condition, e.g. landing approach, the horizontal stabilizer may be repositioned. Repositioning of the horizontal stabilizer allows the control force applied to the elevator to be reduced. In addition, the elevator may be centered about a new pitch axis relative to the horizontal stabilizer, to allow the elevator a maximum amount of deflection both above and below the new pitch axis when responding to pitch command signals.
Various systems for controlling aircraft flight control surfaces and trim tab surfaces have been disclosed. In some aircraft flight control systems the elevator is used to make all pitch command changes and then the horizontal stabilizer is used to trim the elevator control back to neutral. For example, in U.S. Pat. No. 4,382,282--by Graham, there is disclosed an automatic trim system for a stabilizer wherein when established threshold limits for the deflection of the elevator relative to the stabilizer have been reached, a trim motor is automatically actuated to move the horizontal stabilizer so that the angular deflection between the stabilizer and the elevator is reduced in order to reduce aerodynamic drag.
In Lehfeldt U.S. Pat. No. 3,921,941, there is disclosed an apparatus for adjusting flight parameters such as aircraft airspeed and altitude while the aircraft is under control of an autopilot.
A control system for reducing airfoil flutter is disclosed in U.S. Pat. No. 3,734,432--by Low, wherein leading edge and trailing edge control surfaces are deflected in accordance with a predetermined control law to counteract bending and torsional motion of the airfoil.
Kaufman, U.S. Pat. No. 3,369,161 discloses an apparatus for preventing a hardover maneuver in an aircraft by mechanically limiting the range and velocity of operation of a servomotor which positions the control surfaces of the aircraft.
U.S. Pat. No. 3,236,478--by Adams, et al, discloses a flight control system employing normal accelerometer feedback signals for reducing aircraft response to gusts and including signal cancellation means for generating a signal which cancels the portion of the sensed acceleration to which a gust alleviator responds.
In patents by Olhausen, U.S. Pat. Nos. 3,215,374 and 3,240,447, there are disclosed control systems for an aircraft having a canard configuration wherein a compensating change in lift is effected without a change in aircraft pitch moment by generation of a pitching moment in the canard control surface which balances the pitching moment of a primary control surface of the aircraft.
Smith, U.S. Pat. No. 3,137,459, discloses a servo closed loop control system for stabilizing the operation of an autopilot for a missile wherein the actual inflight performance of the missile is compared with preselected reference signals to generate an error signal which is further processed to generate a command signal to the missile's flight controls.
An aircraft autopilot system is disclosed in Bond, U.S. Pat. No. 3,094,299, the autopilot system including pitch control function wherein elevons are operated at or near the center position by incorporation of a trimming unit which achieves deflection of a trimming surface at a rate proportional to the average displacement of the flight control surfaces, and whereby actual trimmer displacement is proportional to the integral of the flight control surface displacement.
An aircraft flight control system is also disclosed in Hecht, U.S. Pat. No. 3,058,700 wherein a signal output to a flight control yaw channel is damped in proportion to the magnitude of that signal thereby preventing excessive loading on the aircraft rudder.
U.S. Pat. No. 2,949,259--by Bell, discloses a system for regulating a flight control surface repositioning servo motor wherein the position of a control surface trim tab is used as an indication of the aircraft's center of gravity in order to regulate the torque developed by the servomotor.
In Curry, U.S. Pat. No. 2,568,719, an aircraft control system is disclosed wherein a common signal is utilized to both control a main motor operating a flight control surface in order to correct for short term transient errors, and to operate a trim motor connected to trim tab attached to a corresponding flight control surface to correct for any sustained or long period errors in the position of the flight control surface.